This invention relates to electromagnetically actuated electrical switches and more particularly to such switches having contact elements sealed in an enclosing envelope.
Sealed contact switches are well known in the electrical arts and have long found extensive application in electrical systems for performing a wide range of switching functions. A typical dry reed form of such switches comprises a pair of overlapping reed springs of a magnetically responsive and electrically conductive material suspended at their ends by an envelope, usually glass, in which they are sealed. A winding encircling the envelope is energizable to generate a magnetic field for actuating the reed springs to control the electrical circuit in which the switch may be connected. Such dry reed switches have served well in particular circuit applications; however, the character of their contact surfaces renders them essentially current limited. Minute imperfections on the contact surfaces reduce the areas of electrical connection to less than the entire surface of the contact. As a result, current of a magnitude beyond a predetermined limit tend to cause melting at the contact areas, which, in turn, increases the tendency of contacts to stick closed after actuation. To counteract this tendency, greater reed spring retractile forces are required which increases the amount of magnetic flux and coil current required to operate the switch.
Where larger current carrying capacities are required, this problem has been largely overcome by the employment of the well-known mercury-wetted relay. Typically, the surfaces of the opposing contacts of this form of sealed relay are coated with a film of mercury. When these surfaces meet upon energization of the relay, electrical connection is uniformly established over the entire contact surfaces. As a result, the magnitude of the currents carried by the relay required to cause contact surface melting is greatly increased. As a further and important result, the current values necessary to actuate the relay are substantially reduced. Since the force required to separate the contacts need not contend with contact melting, the stiffness of the armature may be substantially less than that of the dry reed relay counterpart. The lower limit of armature stiffness of a mercury-wetted relay is determined only by the requirement that the armature restoring force be sufficient to overcome the mercury surface forces and hence rupture the mercury bridge between the contacts. This lower armature stiffness therefore allows mercury-wetted switches to require smaller amounts of magnetic flux for their operation than their dry reed counterparts while having substantially improved load switching capability.
This much higher switching capability makes mercury-wetted contacts very useful in conjunction with solid state circuitry to serve as an interface device capable of handling load currents and voltages which would destroy standard semiconductor devices. However, existing mercury-wetted relays suffer from one or both of two deficiencies, namely, that they are either incompatible with standard logic in size or in current and voltage required to operate the relay. The structural geometry is of particular importance when the miniature size of the relay is considered. In order, for example, to provide a relay more compatible in size with current solid state devices, one having overall external dimensions of 1.0 inches in length, 0.300 inches in width, and 0.275 inches in height is contemplated. It will be appreciated that dimensions of this order impose serious constraints on the possible internal structural arrangements of the relay available to reduce flux path reluctance.
It is accordingly an object of this invention to provide a new and novel miniature mercury-wetted relay construction offering a greater operate sensitivity and higher dielectric and load switching capabilities than hitherto possible in a size compatible with current solid state devices.
Another object of this invention is the reduction of magnetic flux return path reluctance in a miniature mercury-wetted relay.